During hematopoiesis, descendants of hematopoietic stem cells (HSC) become committed to differentiate along specific cell lineages, eventually acquiring the characteristics of terminally differentiated cells. The identification and isolation f HSC has advanced substantially over the past few decades, however, the mechanisms that regulate stem cell lineage commitment and differentiation are still obscure. Steady-state hematopoiesis relies on the tight regulation of apoptosis pathways for the development, maintenance and function of the hematopoietic system. In Philadelphia chromosome negative myeloproliferative neoplasms (Ph- MPN), dysregulation of cell death pathways is frequently observed, implicating cell death factors as potential initiators of disease and targets for therapeutic intervention. Recent studies have highlighted the importance of apoptosis repressor with caspase recruitment domain (ARC) in human malignancies, however, a functional role of ARC in normal hematopoiesis has not been investigated. We hypothesize that ARC plays a critical role in normal blood stem cell and myeloid differentiation. Utilizing an ARC-deficient (ARC -/-) mouse model we will investigate ARC function in hematopoiesis. Our preliminary studies revealed that aged ARC -/- mice exhibit anemia and thrombocytopenia. Investigation of ARC -/- spleens showed splenomegaly with a tri-lineage expansion of myeloid cells, megakaryocyte hyperplasia, and an increase in immature cells consistent with myeloproliferation and extramedullary hematopoiesis. ARC -/- bone marrow revealed increased reticulin staining and reduced cell numbers consistent with marrow fibrosis. The hematopoietic findings in ARC -/- mice are consistent with a Ph- MPN resembling primary myelofibrosis. Further analysis showed expansion of the HSC compartment in ARC -/- bone marrow and in vitro assays revealed altered HSC differentiation capacity in methylcellulose colony assays. Additionally, the myeloproliferative disease in ARC -/- mice is transplantable into lethally irradiated congenic recipients. This projects aims to elucidate the role of ARC in HSC function, myeloid differentiation, and Ph- MPN. To characterize ARC in lineage commitment of stem and progenitor cells, as well as, myeloid differentiation and Ph- MPN cells, we will utilize genetic murine models, stem cell transplant assays, Ph- MPN patient samples and restoration of ARC levels in vivo and in vitro. To elucidate the mechanism of ARC, we will use transcriptional profiling to guide our investigation and identify molecular pathways critical to ARC function.